1. FIELD OF THE INVENTION
The present invention relates generally to equipment and methods for testing material specimens, and more specifically to an apparatus and method for forming concrete specimens for comprehensive strength testing of ACI (American Concrete Institute) xe2x80x9cfield curedxe2x80x9d cylinders. These cylinders are used primarily to insure proper concrete curing for the initiation of early form work removal (i. e., stripping of forms) during construction. The present equipment or apparatus is set into the concrete being poured, and forms a plurality of specimen cylinders meeting the requirements for ASTM testing. The test specimens formed by the present apparatus and method cure in situ, thus providing accurate and representative specimens of the remainder of the concrete from which the specimens were taken. Nearly all of the present apparatus is reusable, thus cutting testing expenses considerably.
2. DESCRIPTION OF THE RELATED ART
It is a standard practice in the construction industry, to test samples of various materials used during construction. This is especially true of concrete, where many field cured specimens are generally required when structural concrete is poured. These specimens are taken from a number of locations across the element or slab when it is poured.
The testing of such specimens is such a standard practice, and so regulated, that the American Society for Testing and Materials (ASTM) has developed certain criteria for the formation and testing of such concrete test specimens. One of the most critical criterion is that the field cured test specimens represent accurate samples of the cured concrete slab. This is critical, as concrete is exothermic (i. e., gives off heat) as it is curing, due to the heat of the hydration reaction which occurs. Accordingly, ASTM requires that field cured specimens which are cast separately from the slab, be treated during curing to closely approximate the heat developed during the curing process in the more massive slab. If the specimens are not in situ, this is difficult to achieve. Also, the specimens must be kept moist until the cure is complete, just as in the case of the poured slab. Any significant variation in the procedure can result in the specimens being an inaccurate representation of the actual qualities of the cast slab they are supposed to represent.
Accordingly, a need will be seen for a means of providing in situ concrete test specimens by means of a largely reusable apparatus. The only portion of the apparatus which is not reusable, comprises an inexpensive plastic cylinder in which the test specimens are cast. The remainder of the device, including the larger container used to encapsulate the smaller test specimen containers during pouring and curing, is removable from the poured slab or foundation after curing. Yet, the sample cylinders formed by means of the present apparatus, are surrounded by the concrete slab during the cure and receive the same heat from the exothermic reaction as does the remainder of the concrete in the slab. The result is a very accurate representation of the actual condition of the concrete of the cured slab, with the largely reusable apparatus resulting in relatively low costs for the test specimens. A method of forming such test specimens in accordance with the apparatus of the present invention, is also disclosed.
A discussion of the related art of which the present inventor is aware, and its differences and distinctions from the present invention, is provided below.
U.S. Pat. No. 3,176,053 issued on Mar. 30, 1965 to Joseph R. Di Stasio, titled xe2x80x9cMethod For Obtaining Test Cores,xe2x80x9d describes an apparatus much like that described in ASTM C873-94, in which outer and inner cylinders are set in the concrete slab at the time of pouring the slab. The outer cylinder remains in place in the concrete after it cures, with the single inner cylinder and its specimen being removed from the outer cylinder. Di Stasio describes his outer cylinder as being formed of materials such as galvanized sheet steel or plastic, and while a very few such cylinders would not add appreciably to the cost of a project, a very large poured concrete project could require thousands of specimens spread throughout the slab. Leaving a major part of the test apparatus buried in the concrete in such a situation where thousands of tests may be required, obviously adds considerably to the cost of the project. Moreover, it is noted that Di Stasio configures his outer and inner containers to leave an air gap between the two. The inner container is thus somewhat insulated from the larger exterior mass of concrete, and the heat of its hydration reaction. The present apparatus and method does not provide any air gaps between the concrete slab and the test specimen apparatus, thus providing a more representative cure and a more accurate specimen.
U.S. Pat. No. 3,541,845 issued on Nov. 24, 1970 to Peter Kierkegaard-Hansen, titled xe2x80x9cMethod For Testing The Strength Of The Material Of Cast Structures, Particularly Concrete Structures,xe2x80x9d describes a reusable test device in which a portion of the device is cast into the concrete and a tension rod is threaded into the device through a hole formed in the concrete. The rod is used to apply tension to the part which is cast into the concrete, until the assembly breaks loose. The tensile force required to break the cast-in part loose, indicates the strength of the concrete. This test method is opposite that described in ASTM C873-94 and the present disclosure, in which compressive force is applied to a specimen removed from the slab after curing.
U.S. Pat. No. 3,595,072 issued on Jul. 27, 1971 to Owen Richards, titled xe2x80x9cConcrete Testing Means,xe2x80x9d describes another tensile testing method and apparatus, similar to that of the Kierkegaard-Hansen U.S. Patent discussed immediately above. Richards uses a hydraulically actuated tension device to pull a rod which has been threaded into an anchor device in the concrete. While the entire apparatus is recoverable from the concrete (assuming the test is continued to destruction), the tensile test is not in accordance with ASTM methods for compressively testing specimens removed from a concrete slab, whereas the present invention provides a means for forming specimens which are accurate representations of the remainder of the concrete slab and which comply dimensionally with current ASTM test equipment.
U.S. Pat. No. 3,861,201 issued on Jan. 21, 1975 to Franz Kaindl, titled xe2x80x9cMethod And Apparatus For Early Strength Testing Of In-Place Concrete,xe2x80x9d describes the use of a screen installed before the concrete is poured, for screening large aggregate pieces in order to provide a more uniform specimen. A base may be imbedded in the concrete into which a tension rod is installed for tensile strength testing, or the specimen may be compressively tested in place as desired. The test specimen is not cast in place and then removed for testing, as provided by the present invention. While the goal of the Kaindl apparatus, i.e., to provide a uniform representative specimen free of large aggregate pieces which would produce large variation in the results, is appreciated, the test resulting from the Kandl device may not be truly representative.
U.S. Pat. No. 3,974,679 issued on Aug. 17, 1976 to Karim W. Nasser, titled xe2x80x9cAccelerated Concrete Strength Testing,xe2x80x9d describes a testing apparatus which applies pressure and heat to a cylinder(s) of wet concrete. The heating and pressure result in a cure equivalent to a 28 day aging period, in a time of only about five hours. The apparatus is quite complex and costly in comparison to the present in situ specimen apparatus, and in any event, some of the wet concrete mix of the slab being poured must be placed in the mold(s) of the Nasser apparatus in order to have a truly representative sample. Thus, the Nasser apparatus must be located on, or very near, the actual job site; this would appear to pose some problems regarding portability and operation. The present in situ specimen apparatus is placed directly in the slab being poured, with the contents curing with the remainder of the slab, to provide accurate specimens of the concrete slab.
U.S. Pat. No. 4,182,191 issued on Jan. 8, 1980 to Shoji Ikeda, titled xe2x80x9cMethod Of Immediate Estimation Of Compressive Strength Of Concrete Through Quick Hardening,xe2x80x9d describes the addition of an alkali hydroxide and an alkali accelerator to the wet mix to accelerate the curing and aging chemically. The resulting specimen is not chemically identical to the concrete of the poured slab, due to the alkali hydroxide and accelerator additions, whereas the present invention uses exactly the same concrete as used in the rest of that area of the slab. Moreover, the Ikeda process requires that the specimen be poured separately from the slab, rather than poured in situ, as in the present system.
U.S. Pat. No. 4,425,801 issued on Jan. 17, 1984 to Ulrich W. Stoll, titled xe2x80x9cDevice And Procedure For Measuring In Situ Strength Of Concrete And The Like,xe2x80x9d describes a device imbedded in the fresh concrete at the time it is poured. After curing, a torque is applied to a handle extending from the device, with the breakaway force being used to determine the strength of the concrete. Thus, the Stoll device and method relate more to the actual testing of the concrete, rather than to any provision for the formation of test slugs or specimens, as in the present invention. In any event, the Stoll apparatus applies a torque to the test sample, rather than a conventional compression test in accordance with standard ASTM procedures.
U.S. Pat. No. 4,501,153 issued on Feb. 26, 1985 to Ferenc Mehes et al., titled xe2x80x9cTest Machine For Determining Concrete Strength,xe2x80x9d describes a device having a hollow conical section which is imbedded in the wet concrete. After curing, a tensile force is applied to the device, with the separation line occurring across the narrower lower end of the device when the breaking point is reached. Thus, the Mehes et al. device actually tests the concrete in situ, rather than providing an in situ test specimen for testing using a conventional remotely located ASTM test apparatus, as provided by the present invention. The present apparatus and method provides test specimens which are fully compatible with conventional and accepted ASTM compressive testing machines and procedures, unlike the Mehes et al. device.
U.S. Pat. No. 5,677,495 issued on Oct. 14, 1997 to Claude D. Johnson et al., titled xe2x80x9cCompressive Strength Testing Of HPC Cylinders Using Confined Caps,xe2x80x9d describes the provision of caps specially formed for use in testing high performance concrete (HPC). Conventionally, cylindrical concrete specimens are tested under compression to determine their ultimate compressive breaking strength. Caps are placed upon each end of the concrete cylinder, to distribute the compressive forces more uniformly across the ends of the cylinder and reduce splitting or shattering of the edges of the cylinder before maximum compressive pressure is reached. Thus, the Johnson et al. U.S. Patent is directed more to the actual testing of concrete specimens, rather than the forming of such specimens in situ in a reusable apparatus, as is accomplished by the present invention.
British Patent Publication No. 1,413,160 published on Nov. 5, 1975 to Shell International Research Maatschappij B. V., titled xe2x80x9cMethod And Means For Load Testing Open-Ended Piles Penetrating The Soil,xe2x80x9d describes a method and apparatus for testing the solidity of hollow pilings driven into the ground. A compressive force is applied to a plate at the bottom of the piling, and bears against an upper plate anchored within the piling. Shifting of either the lower plate or the piling indicates some instability. Thus, the ""160 British Patent Publication does not disclose any means of providing an in situ concrete specimen for testing, but rather discloses an in situ test for the anchoring of a piling in the soil, unlike the present invention.
Finally, the American Society for Testing and Materials (ASTM) document no. C873-94, titled xe2x80x9cStandard Test Method For Compressive Strength Of Concrete Cylinders Cast In Place In Cylindrical Molds,xe2x80x9d describes such a test method in which a cylindrical shell is permanently imbedded in the wet concrete, with a separate cylindrical container installed within the outer shell. Concrete is poured in and around the assembly and allowed to cure. The inner shell containing the single cured specimen is then removed from the imbedded outer shell after the concrete has cured, and transported for testing. The problem with this ASTM apparatus and process has been noted above, in that standard procedures may require a large number of separate test specimens from a relatively large slab. The large number of outer cylinders which must be left imbedded in the concrete, result in a significantly higher cost for the project. The present apparatus and method recovers everything from the slab, with only a few very inexpensive molds destroyed to recover the specimens cast therein. Moreover, the ASTM procedure is silent regarding any capping or closure of the removed mold, whereas the present apparatus includes caps which are applied to the individual molds in order to retain moisture and protect the specimens until testing. Also, the present apparatus preferably includes a plurality of separate molds in each larger reusable shell or container, thus providing a series of specimens to insure the consistency of the compressive strength results. The present apparatus preferably provides three test cylinders, while only two are required by ASTM and ACI. By providing three test cylinders, any wide variation which appears during the testing can be dealt with. As an example, if the three specimens provided by the present sample apparatus test at 3550 psi, 3520 psi, and 2220 psi, one can be reasonably certain that the actual compressive strength is slightly over 3500 psi, with the low test being an obvious anomaly which may be discarded. By using an average of three specimens from each test point, such variations will show which are not at all apparent if only two samples are taken and one of those specimens produces an anomalous result; the sample producing the erroneous result cannot be determined with only two specimens.
None of the above inventions and patents, either singly or in combination, is seen to describe the instant invention as claimed.
The present invention comprises a reusable, in situ concrete test specimen apparatus, and method for using the apparatus and forming test specimens. The present apparatus essentially includes a relatively large and flexible open outer container which is placed within the area where a concrete slab is to be poured. A series of smaller closable containers is placed within the larger container. The smaller containers rest upon a platform or base which may be lifted from the larger container (along with all cured concrete therein) once the concrete has cured. The smaller containers are filled with the same concrete mix as used to form the remainder of the slab at least in the immediate area, and thus serve as representative specimens of the concrete placed in the slab.
The smaller containers remain capped after the concrete is poured to retain the moisture in the containers for proper curing of the specimens. As the specimens cure in situ, they are subject to the same latent heat developed by the exothermic reaction of the remainder of the slab during cure, and need not be heated by other means. Once the concrete is cured and the specimens removed, the outer flexible container may be removed from the resulting hole, and the hole filled with concrete to close the test void in the slab. The present apparatus is completely reusable, excepting the inexpensive smaller cylindrical containers which are cut away from the cured specimens contained therein for testing those specimens. Thus, the present test specimen forming apparatus and method provides considerable economic advantages over other earlier methods and devices.
Accordingly, it is a principal object of the invention to provide an improved reusable apparatus for forming concrete test specimens in situ, and a method of forming such specimens in situ using the present apparatus.
It is another object of the invention to provide an improved apparatus for forming concrete test specimens, comprising a relatively larger, flexible, open outer container and series of smaller closable containers for placing in an area where a concrete slab is being poured, for filling with a representative specimen of the poured concrete.
It is a further object of the invention to provide an improved apparatus for forming concrete test specimens including means for removing the smaller containers from the larger container after the concrete poured therein has cured.
An additional object of the invention is to provide an improved apparatus for forming concrete test specimens which smaller containers may remain sealed until delivered for testing, and which smaller containers form specimens which are dimensionally acceptable for standard ASTM compressive test equipment and procedures.
Still another object of the invention is to provide an improved apparatus for forming concrete test specimens which larger outer container is removable from the concrete after removal of the smaller containers therefrom.
It is an object of the invention to provide improved elements and arrangements thereof in an apparatus for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.
These and other objects of the present invention will become apparent upon review of the following specification and drawings.